Bedsores, or decubitus ulcers, can be a serious problem in bedridden or wheelchair-bound patients, particularly for people who are paralyzed, emaciated, post-surgical, elderly, or diabetic. The ulcers frequently penetrate through not only the skin, but the underlying muscle and bone as well. With the serious infections that often ensue, pressure ulcers can become life-threatening.
As the elderly population increases with demographic trends, the incidence is likely to increase. The results of National Pressure Ulcer Surveys in the United States from 1989 to 1997 indicate that despite the growth in the wound care and therapeutic surface industries, the incidence of pressure ulcers appears to have increased over this period. It is clear that while new treatment solutions may be relatively effective, their cost precludes their use by the vast majority of caregivers in the settings in which pressure ulcers and other chronic wounds must be managed. Disproportionately, this includes the nursing home, home care, and of course, the overseas markets where resources are limited. The consensus among thought leaders in the international medical community supports the contention that less expensive medical solutions are required generally and urgently. The invention to be described here is intended to fulfill this societal need.
Bedsores, or pressure ulcers, were named because they most commonly develop where tissue pressures are greatest—over the bony prominences, such as the heels, sacrum (tailbone), ischia, greater trochanters, and ankles (external malleoli). At these sites where the pressure on the skin is concentrated, blood flow can be restricted. If nutrient deficit exceeds tissue demand over a given interval, the tissue will start to die locally, resulting in an ulcer.
It is generally recognized that it is important to limit both skin warming and moisture accumulation to effectively combat skin breakdown. This has been embraced by professional bodies and recognized thought-leaders in the wound care medical community.
The normal core temperature of the human body is between 36° and 38° C. Skin temperature typically ranges between about 30° C. and about 34° C., depending on ambient temperature, the amount and type of clothing being worn, the core temperature, and where the skin is located on the body. However, on a typical mattress, seat cushion, seat back, etc., heat is trapped between the body and the covered skin surface and the skin temperature rises rapidly and may reach 35 to 37 degrees C. This small temperature elevation that occurs with the skin in contact with the mattress, seat cushion, etc., has important physiologic effects.
When a patch of skin is warmed beyond a specific level referred to as the “perspiration threshold” of approximately 32 to 34° C., local perspiration in the region increases markedly. The accompanying moisture softens the skin (maceration), which makes it more susceptible to breakdown. The build-up of moisture increases the friction between the skin and the surface materials resulting in increased shear stresses in the tissue. It has also been shown that elevated skin temperature is associated with increased metabolic demand, therefore, researchers have speculated, increasing the susceptibility of the tissue to ischemic injury, particularly when both nutrient supply and metabolite removal are reduced by loading. Generally, tissue metabolic rates increase by approximately 10% for each one degree Celsius increase in temperature. Warmed tissue generates an increased demand for blood supply that can be met when the skin is not under significant load. At interface pressures of 20 or more mm Hg, as occur under the bony prominences on a mattress or seat, blood flow can not be increased to meet this demand and the tissue becomes ischemic. A study demonstrated that skin tissue with reduced blood supply has been shown to be less susceptible to injury when tissue temperatures were slightly reduced. In a second study, identical pressures were applied to the skin tissue of research animals at nearly 300 sites. The skin temperatures at the interface varied between 28 and 36 degrees C. The results showed a very strong positive correlation—nearly perfect, in fact—between skin temperature and degree of skin breakdown.
When skin temperatures are maintained within certain limits, the person or animal is more comfortable. For humans, comfort is optimal when the skin temperature is maintained close to its natural (non or lightly insulated) temperature of 30 to 34 degrees C., even when insulated support conditions are employed. The devices described herein have important medical and non-medical applications. The non-medical applications include most seating and bedding applications, such as mattresses for the home, mattress overlays, tickings, pillowcases or pillows, or seating or seat backs for the office, home, and vehicle markets.
Temporary skin cooling can be accomplished by increasing the heat input required to increase the temperature of the surface. The quantity of heat required to increase the temperature of a given quantity of material by a specific temperature is called the specific heat. The specific heat can be expressed in Joules/kg-degree K. The quantity of heat required to raise the temperature of a given body is referred to as the heat capacity of the body. If a large sample and a small sample are both made of the same material, for example, the larger sample will have a greater heat capacity although both will have the same specific heat. A surface composed of high specific heat material such as silicone gel or fluid, or even a waterbed, will provide temporary cooling because a great deal of the body's heat will flow from the skin, initially at approximately 30 to 34° C. to the surface, initially at 23° C. room temperature. The skin will continue to be cooled as long as the surface remains cooler than the skin. Materials with low specific heat, such as a urethane foam, warm rapidly toward body temperature and therefore cool the skin only very briefly.
In order to provide continuous, steady-state cooling, heat is removed and transferred to the environment or to another system that is external to the surface to be cooled. A need exists for non-powered, or, stated otherwise, self-powered, relatively inexpensive devices to provide steady state cooling at the level of the expensive, externally powered LAL surfaces currently in use. It is valuable to develop such a device, whether powered or not, that provides cooling without spreading airborne pathogens from the occupants' skin surface into the common environment, as appears to be the case with low air-loss surfaces due to their reliance on high volume blowers or air-pumps. Adding a small powered thermoelectric module to enhance heat withdrawal by the invention improves performance in all environments with greatly reduced overall air-flow, and hence, reduced risk of spreading air-borne infection
The likelihood of bedsore formation is reduced by lowering tissue metabolic rate (and therefore reducing the nutrient-deficit in tissue that is pressure-loaded and subject to reduced blood flow) and by limiting local perspiration, which weakens the outer skin layer (the stratum corneum) over time. These inventions may be used as an aid in the prevention of bedsores or other skin ulcers.
Moderate cooling of the skin during support (from 35° C. to 37° C. down to the 30° C. to 34° C. range) also makes the user more comfortable. This would be true in both bedding and seating applications, in medical and consumer environments. When used over broader areas of the skin, the cooling devices, whether over the skin (blanket or duvet-insert) or under the skin (mattress or seating overlays) may be useful as an aid in combating fever in both home and medical environments. The proposed inventions therefore have not only medical applications, but applications in a multitude of general consumer niches as well.